Contamination control when growing yeasts

Abstract

A method for contamination control when growing yeasts is provided. Bacterial contamination is controlled by using urea as the primary nitrogen source while simultaneously limiting the amount of nickel available to contaminating bacteria. Bacteria require nickel as a cofactor for urease enzymes in order to use urea for growth while yeasts do not require nickel as a cofactor for any enzymes. Nickel is limited by using metals in heat exchangers that do not leach nickel. Ethyl carbamate is limited by using a carbon/nitrogen ratio that consumes all urea during fermentation. After fermentation completes, yeast is recycled using centrifugation, enabling use of high concentrations of yeast to reduce fermentation time from 48 hours to 12 hours and to eliminate bacterial contamination from growth on free amino nitrogen.

I claim: 
     
         1 . A method for contamination control when growing yeasts, said method comprising growing yeasts for a fermentation time at a starting pH in a fermentation broth comprising a sugar solution, free amino nitrogen, urea, a mineral source, yeasts, and contaminating bacteria,
 wherein the amount of nickel in said fermentation broth is less than 1 mg/kg during the entirety of said fermentation time,   wherein said urea is introduced to said fermentation broth via fed-batch feeding over time, with said fed-batch feeding commencing after said yeasts consume more than 90% of said free amino nitrogen in said fermentation broth, then controlling said fed-batch feeding such that the pH of said fermentation broth does not rise, due to addition of said urea, during said entirety of said fermentation time,   wherein said sugar solution contains sugars selected from the group consisting of sucrose, glucose, fructose, mannose, galactose, lactose, xylose, maltose, and combinations thereof,   wherein said sugar solution has a starting concentration of sugars greater than 50 g/L,   wherein said free amino nitrogen has a starting concentration between 10 mg/L and 400 mg/L of nitrogen,   wherein said yeasts have a starting concentration of more than 50 g/L wet weight,   wherein said contaminating bacteria have a starting concentration of less than 10 7  CFU/mL,   wherein said fermentation time is between 2 hours and 18 hours, during which said yeasts grow by metabolizing said sugars, said free amino nitrogen, said urea, and said mineral source in said fermentation broth, wherein said yeasts produce metabolites in said fermentation broth, and wherein said metabolites are selected from the group consisting of ethanol, carbon dioxide, glycerol, acetaldehyde, and combinations thereof,   wherein a portion of said yeasts is separated from said fermentation broth after said fermentation time to produce recycled yeasts, and   wherein said recycled yeasts are not washed with acid, a fraction of said recycled yeasts is re-used in subsequent cycles as said yeasts, and the remaining fraction of said recycled yeasts is used as a yeast cream co-product.   
     
     
         2 . The method of  claim 1 , wherein said sugar solution is produced from grain by first grinding said grain to produce a ground grain, forming a slurry of said ground grain and alpha-amylase enzymes and liquifying said slurry to a dextrin solution containing dextrin at a temperature above 80° C., saccharifying more than 95% of said dextrin to provide a saccharified solution using glucoamylase enzymes at a temperature between 50° C. and 65° C., and centrifuging said saccharified solution to produce a supernatant of said sugar solution and a co-product cake. 
     
     
         3 . The method of  claim 1 , wherein said sugar solution is produced from solutions selected from the group consisting of sugarcane juice, clarified sugarcane juice, sugarcane molasses solution, raw sugar solution, sweet sorghum juice, clarified sweet sorghum juice, sugar beet juice, clarified sugar beet juice, sugar beet molasses solution, sucrose solution, glucose solution, fructose solution, mannose solution, galactose solution, lactose solution, xylose solution, maltose solution, and combinations thereof. 
     
     
         4 . The method of  claim 1 , wherein said yeasts are selected from the group consisting of  Saccharomyces cerevisiae, Cyberlindnera jadinii, Kluyveromyces marxianus, Yarrowia lipolytica , and combinations thereof. 
     
     
         5 . The method of  claim 1 , wherein said contaminating bacteria is  Lactobacillus fermentum.    
     
     
         6 . The method of  claim 1 , wherein said fermentation broth contains said ethanol which is separated, using distillation, to produce a hydrous ethanol product and distillation stillage. 
     
     
         7 . The method of  claim 1 , wherein the step of separating said recycled yeasts from said fermentation broth is selected from the group consisting of centrifugation, filtration, sedimentation, foam fractionation, and combinations thereof. 
     
     
         8 . The method of  claim 2 , wherein said co-product cake is dried to less than 12 wt % moisture to produce a dried animal feed product. 
     
     
         9 . The method of  claim 6 , wherein a portion of said distillation stillage is mixed with water to form said slurry, and the remainder of said distillation stillage is concentrated to produce an animal feed syrup. 
     
     
         10 . The method of  claim 2 , wherein said grain is selected from the group consisting of corn, wheat, rice, sorghum, and combinations thereof. 
     
     
         11 . The method of  claim 1 , wherein said fermentation broth is in operable communication with a heat exchanger, wherein said heat exchanger is a plate heat exchanger comprising titanium heat exchange plates, and wherein said titanium heat exchange plates contain less than 1 g/kg nickel. 
     
     
         12 . The method of  claim 1 , wherein said fermentation broth is in operable communication with a heat exchanger, and wherein said heat exchanger is a plate heat exchanger comprising stainless steel grade 316 heat exchange plates. 
     
     
         13 . The method of  claim 1 , wherein said fermentation broth is in operable communication with a heat exchanger, wherein said heat exchanger is a spiral plate heat exchanger comprising titanium heat exchange plates, and wherein said titanium heat exchange plates contain less than 1 g/kg nickel. 
     
     
         14 . The method of  claim 1 , wherein said fermentation broth is in operable communication with a heat exchanger, and wherein said heat exchanger is a spiral plate heat exchanger comprising stainless steel grade 316 heat exchange plates. 
     
     
         15 . The method of  claim 1 , wherein said fermentation broth is cooled by evaporative cooling. 
     
     
         16 . The method of  claim 15 , wherein said fermentation broth is sprayed into a space to form a spray, wherein air at less than 100% relative humidity is circulated through said space, wherein said air evaporates water from said spray, and wherein said spray is returned to said fermentation broth at a reduced temperature. 
     
     
         17 . The method of  claim 6 , wherein the ratio of the amount of said carbon source to the amount of said urea is selected such that essentially no urea remains in said fermentation broth before said hydrous ethanol product is separated from said fermentation broth by said distillation. 
     
     
         18 . The method of  claim 1 , wherein said fermentation broth is oxygenated to maintain an oxygen level sufficient to support aerobic growth of said yeasts. 
     
     
         19 . The method of  claim 18 , wherein said fermentation broth is oxygenated using a method selected from the group consisting of air sparging, foam fermenting, mechanical agitation, and combinations thereof.